1. Field of the Invention
The present invention relates to manufacture of a semiconductor device, and more particularly, to a photomask used in a photolithography process for manufacturing a semiconductor device, a method for manufacturing the same and a method for measuring optical characteristics of a wafer exposure system, such as focus, lens aberration, etc., using the photomask during the operation.
2. Description of the Related Art
Recently, as a critical dimension (CD) in a device during manufacture of a semiconductor device becomes strict, patterns to be exposed are greatly influenced by distributions of lens focus and aberrations. Accordingly, understanding and controlling the distributions of the lens focus and aberrations during operation of the exposure system is recognized as being very important.
Until now, the methods for measuring or detecting a degree of focus of the exposure system have concentrated on measuring a CD of a real pattern on a real wafer according to the defocus of the exposure system using a scanning electron microscope, or measuring shifts of the patterns according to the defocus using overlay measuring equipment. However, it is known to be very difficult to precisely measure focus of the exposure system with the method for measuring a CD of a real pattern using a scanning electron microscope. That is, it is possible to measure a CD of a real pattern using a scanning electron microscope, but it is difficult to determine if the CD measured by this method represents the exact degree of a focus of the exposure system.
In addition, it is known that the method for measuring an overlay can measure a degree of focus of the exposure system relatively precisely, but presents difficulties in manufacturing a photomask for patterns observed with the overlay measurement equipment. Patterns for measuring an overlay are manufactured much larger than the patterns for the real devices. Accordingly, in the case of manufacturing a photomask corresponding to the patterns for measuring an overlay, a considerably large pattern should be manufactured on the photomask. It is known that the adhesiveness between a quartz blank and photoresist for a photomask is poor. Therefore, since a photoresist pattern should cover a large region on the quartz blank to manufacture a photomask including a large pattern, it occurs quite often that photoresist patterns are lifted when the photoresist patterns are exposed or developed.
Therefore, there is a need for a method for measuring or detecting optical characteristics such as focus or lens aberrations of the exposure system during manufacture of a semiconductor device, and a photomask used in the method.